The beginning involves Holmes firing his gun indoors. This is something he had been known to do, and it's funny! Mr. Wilson faints when he hears the shots, which fits his character.

Then the movie follows the story pretty closely. It begins with Mr. Wilson telling them about the advertisement, except that we see the actual events acted out.

Then Watson and Holmes visit the shop. Here is a major difference, because Watson engages the suspect in idle chitchat about the curiosities in the store to allow Holmes to look around. In the story, Holmes just asks for directions.

“Thank you,” said Holmes, “I only wished to ask you how you would go from here to the Strand.” (p. 13)

Personally, I find the Watson character pretty funny in this version. I enjoyed the scene in the shop. After that, the movie and book were pretty similar, except at the end when Holmes denies shooting the gun, something the police find hard to accept anyway.